Orthogonal modulation method, which transmits transmission information by allocating the transmission information to orthogonal signals, is a method, that is capable of suppressing bit error rates occurring in a white Gaussian noise transmission channel when the number of the dimensions of the utilized orthogonal signals is large, and that is particularly useful in a communication system that requires high power efficiency. The orthogonal signals used in orthogonal modulation method can be configured as desired as long as the orthogonal signals are mutually orthogonal. For example, orthogonal frequency shift keying (FSK), which selects and transmits a single sequence associated with the transmission information from among sequences formed by combining sine waves of different frequencies, can be regarded as one type of orthogonal modulation method.
Although an orthogonal matrix is used in orthogonal modulation method to define the orthogonal signals, a method of extension of orthogonal modulation method is proposed as bi-orthogonal modulation method that uses an orthogonal matrix and, for example, a matrix that is complementary to the orthogonal matrix, to define the orthogonal signals (Non-Patent Literature 1). Here, the complementary matrix is a matrix, for example, in the case in which each of the elements of the orthogonal matrix is a binary value of “0” or “1”, in which the “0” and “1” values of each of the elements are reversed. In bi-orthogonal modulation method, the number of orthogonal signals capable of allocation for the transmission information doubles, and the information amount capable of transmission by a single orthogonal signal doubles, and thus frequency utilization efficiency can be improved.
For the portion defined using the matrix, among the orthogonal signals of the aforementioned bi-orthogonal modulation method, that is complementary to the orthogonal matrix, the signal may be regarded as a signal having an inverse phase with respect to the signal of the orthogonal signal defined by the orthogonal matrix acting as the source of frequency utilization efficiency thereof, that is to say, may be regarded as a signal having phase-rotated by π. Further, due to there being no requirement for restricting the phase rotation to π, bi-orthogonal modulation method can be generalized as the method of transmitting transmission information by use of orthogonal signals defined by a single source orthogonal matrix, and by a new orthogonal matrix that phase-rotates by a predetermined phase rotation amount the source orthogonal matrix.
Furthermore, many more orthogonal signals can be further defined by producing a plurality of orthogonal matrixes that are phase-rotating by different phase rotation amounts relative to the single orthogonal matrix, and in this case, the number of sequences capable of allocation for the transmission information increases in response to the number (types) of phase rotation amounts of the phase rotations of the orthogonal matrix. In this manner, the number of orthogonal signals in bi-orthogonal modulation method is determined according to the number of dimensions of the source orthogonal matrix and the number (types) of phase rotation amounts of the phase rotations of the source orthogonal signal, and the information amount, such as the bit count, that can be transmitted using a single symbol (one orthogonal signal) is determined according to the number of orthogonal signals. When the number of the orthogonal signals increases, the transmittable information amount using a single symbol can increase, and frequency utilization efficiency can increase.